Abstract : The catalytic repertoire of baker's yeast has been expanded to include enantioselective Baeyer-Villiger oxidations. To create this catalyst, the Acinetobacter sp. (NCIB 98%l) cyclohexanone monooxygenase gene was cloned into a yeast expression plasmid and this vector was used to transform baker's yeast (Saccharomyces cerevisiae). Whole cell-mediated Baeyer-Villiger reactions were carried out on a 1.0 mmole scale and several cyclic ketones were converted in 20-30 hours into the corresponding lactones in isolated yields of 54-83%. Under our reaction conditions, ketone reduction caused by the host yeast reductases constituted only a minor side-reaction. In the first phase of this work, reaction conditions for the Baeyer-Villiger yeast-mediated oxidation of cyclohexanone were optimized. The second phase v involved the oxidation of prochiral 4-substituted cyclohexanones by our engineered yeast. The enzyme-mediated oxidation of these substrates afforded lactones with very high enantioselectivities. while we did encounter some problems with substrate solubilities, these were easily solved by the addition of stoichiometric amounts of beta cyclodextrin. In the third and final phase, we studied the oxidation of racemic 2-substituted cyclohexanones. As the size of the substituent increased, we found that the kinetic resolutions improved dramatically. In all cases, the (S)- ketone was more reactive than the antipode. This kinetic resolution allowed us to isolate both enantiomerically enriched ketones and lactones, generally with ee values of greater than 98%. Recovered yields of the lactone products varied from 54 to 79%. In conclusion, we have created a simple reagent capable of performing a chiral Baeyer-Villiger oxidation.